Method and device for plasma treatment of moving metal substrates

ABSTRACT

The invention relates to a method of treatment, in particular cleaning and/or heating, for a metal substrate ( 1 ) fed in a substantially continuous manner through a vacuum chamber ( 3 ), having a treatment zone in which an electric discharge ( 10 ), i.e. a plasma, and a magnetic field are produced in a gas maintained at a pressure below atmospheric pressure between at least the substrate ( 1 ), acting as an electrode, and at least one counter-electrode ( 9 ) to enable the substrate ( 1 ) to be bombarded by the ions produced in the electric discharge ( 10 ). This method is characterised in that a confining magnetic induction field is produced entirely around the substrate ( 1 ) in the treatment zone so that the electric discharge ( 10 ) is also confined entirely around the substrate ( 1 ) inside this treatment zone by the confinement of electrons released in the electric discharge ( 10 ).

[0001] The present invention relates to a treatment method, inparticular cleaning and/or heating by means of an electric discharge,i.e. by plasma, for metal substrates such as products in the form ofwires, tubes, beams, strips and/or sheets. During this process, thesubstrate to be treated is displaced in a given direction through anenclosure with a treatment zone, in which an electric discharge isgenerated between a counter-electrode and the substrate, close to thesurface of the latter.

[0002] This process enables a layer of contamination to be removed fromthe substrate, such as superficial metal oxide and superficial carbonfor example, in order to assist the adhesion of a coating applied to theproduct subsequently using vacuum deposition technology.

[0003] This process also enables the substrate to be effectively heatedand can therefore be used as a means of annealing metal products. It maybe used for substrates of mild steel, stainless steel, aluminium, copperand other metals.

[0004] Known methods of cleaning or heating a metal product by plasmahave various disadvantages. For example, in the methods known from theprior art, it is not possible to treat all the external surfaces of theproduct in a single operation.

[0005] Specifically when treating a flat metal strip with a top face anda bottom face, the two faces are treated in succession by generating aplasma close to the two faces one after the other. A significantdrawback of this approach is that the face treated during the first stepcan become contaminated again whilst the second face is being treatedduring the second step.

[0006] In order to be able to treat the metal strip efficiently, amagnetron discharge is generated on its surface by arranging systems ofmagnets to the rear of the surface to be treated.

[0007] Another major disadvantage of the known method is the problem ofadapting the size of the systems of magnets to the width of the stripand the fact that it is impossible to treat wires and tubes using thistechnique.

[0008] One of the main objectives of the present invention is to proposea method which enables disadvantages of this nature to be overcome.

[0009] To this end, the method proposed by the invention consists ingenerating a confining field of magnetic induction entirely around thesubstrate in the region to be treated so that the electrical dischargeis confined to the area entirely around the substrate due to theconfinement of electrons released in the discharge, to enable ahigh-density plasma to form all around the substrate.

[0010] Advantageously, a magnetic induction field is created in saidtreatment zone substantially parallel with the axis along which thesubstrate is fed through this treatment zone, thereby enabling amagnetron discharge to form around the substrate by circulatingelectrons from the discharge along trajectories extending around thesubstrate.

[0011] In one particularly advantageous variant of the method proposedby the invention, at least one magnetic mirror is created at leastpartially around the substrate so that the electric discharge is mainlyconfined to the treatment zone around the substrate by this magneticmirror.

[0012] For practical purposes, a magnetic induction field is createdwhich extends substantially transversely to the direction ofdisplacement of the substrate and which has a minimum value close to thesurface of the substrate where the electrical discharge is confined.

[0013] In particular, the intensity of this magnetic field increasesfrom the substrate to said magnetic mirror by a factor of at least two.Consequently, as they move away from the substrate, the electrons arereflected and sent back towards the substrate, the effect of which is toconfine the electric discharge around the substrate.

[0014] In another advantageous embodiment, at least two magnetic mirrorsare created through which the substrate is passed, and which define theinlet and the outlet of the treatment zone. These magnetic mirrorsenable the electric discharge to be confined in a directionsubstantially parallel with the axis along which the substrate is fedthrough the treatment zone.

[0015] A magnetic induction field is formed by these magnetic mirrorssubstantially parallel with the feed direction of the substrate. Theintensity of this induction field decreases at an increasing distancefrom each mirror towards the centre of the treatment zone.

[0016] The invention also relates to a device for treating a metalsubstrate, in particular for cleaning and/or heating a metal substrate,comprising a vacuum chamber provided with an inlet opening and an outletopening for the substrate. The substrate may be fed through this vacuumchamber, which constitutes a treatment zone in which the electricdischarge is created, in an essentially continuous manner. The devicehas magnetic means for confining electrons produced in the electricdischarge and has at least one counter-electrode disposed facing thesubstrate, which acts as an electrode.

[0017] The device proposed by the invention is characterised in that themagnetic confinement means are disposed relative to the substrate sothat the ions produced in the electric discharge can be kept entirelyaround the substrate and thus bombard its surface. More specifically,the magnetic confinement means are provided at least partially aroundthe treatment zone.

[0018] At least one magnetic mirror is advantageously disposed aroundthe treatment zone through which the substrate is fed in such a way thatit reflects and sends back towards the substrate the electrons leavingthe latter, thereby confining the electric discharge to this zone aroundthe substrate.

[0019] The magnetic confinement means preferably comprise at least onesolenoid around the treatment zone, the axis of which is substantiallyparallel with the feed direction of the substrate. The cross-section ofthe solenoid perpendicular to its axis may generally be of any shape,for example circular in the case of a cylindrical solenoid orrectangular in the case of a parallelepipedic solenoid.

[0020] In one particularly practical embodiment of the invention, thedevice has at least two magnetic mirrors between the inlet opening andthe outlet opening, bounding the treatment zone in the direction inwhich the substrate is fed.

[0021] Other details and features of the invention will become clearfrom the description below, outlining specific embodiments of theinvention in the form of examples, which are not intended to berestrictive, with reference to the appended drawings.

[0022]FIG. 1 is a schematic diagram showing a cross-section through afirst embodiment of the device proposed by the invention, parallel withthe axis along which the product is fed.

[0023]FIG. 2 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is fed through the device illustratedin FIG. 1.

[0024]FIG. 3 is a schematic diagram showing a cross-section through asecond embodiment of the device proposed by the invention, parallel withthe axis along which the product is fed through the device.

[0025]FIG. 4 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is fed through the device illustratedin FIG. 3.

[0026] La FIG. 5 is a schematic diagram showing a cross-section througha third embodiment of the invention, parallel with the axis along whichthe product is fed.

[0027]FIG. 6 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is displaced through the deviceillustrated in FIG. 5.

[0028]FIG. 7 is a schematic diagram showing a cross-section through afourth embodiment of the device proposed by the invention, parallel withthe axis along which the product is fed.

[0029]FIG. 8 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is displaced through the deviceillustrated in FIG. 7.

[0030]FIG. 9 is a schematic diagram showing a cross-section through afifth embodiment of the device proposed by the invention, parallel withthe axis along which the product is fed.

[0031]FIG. 10 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is displaced through the deviceillustrated in FIG. 9.

[0032]FIG. 11 is a schematic diagram showing a cross-section through asixth embodiment of the device proposed by the invention, parallel withthe axis along which the product is fed.

[0033]FIG. 12 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is displaced through the deviceillustrated in FIG. 11.

[0034]FIG. 13 is a schematic diagram showing a cross-section through aseventh embodiment of the device proposed by the invention, parallelwith the axis along which the product is fed.

[0035]FIG. 14 is a graph plotting the value of the component of themagnetic induction field in the vicinity of the substrate in thedirection in which the substrate is displaced through the deviceillustrated in FIG. 13.

[0036]FIG. 15 is a detail in cross-section, parallel with the axis alongwhich the product is fed, this diagram providing a schematicillustration of the discharge, indicating the behaviour of the electronsbetween two magnetic mirrors.

[0037]FIG. 16 is a schematic diagram showing a cross-sectionperpendicular to the direction of displacement of the product,illustrating how a magnetron discharge is formed around the product.

[0038] The same reference numbers are used to denote identical orsimilar elements in the different drawings.

[0039] The method proposed by the invention consists in creating aplasma in the proximity of the surface of a substrate to be cleaned orheated, in a gaseous mixture containing, for example, one or more of thefollowing elements: Ar, He, H₂, O₂ or N₂, or hydrocarbon compounds. Thegas is maintained at a pressure P below atmospheric pressure, so as togenerate radicals and ions enabling the substrate to be cleaned and thetemperature to be raised. The pressure P may be between 10⁻² Pa and 1000Pa, for example.

[0040] By creating a plasma at a high density n in the gas maintained atlow pressure, this process enables highly kinetic substrates to becleaned or heated. This plasma density n generally corresponds to anelectronic density of between 10¹⁰ cm⁻³ and 10¹² cm⁻³. Because theplasma is at a high density, a high level of power can be dissipated atthe surface of the substrate, mainly in the form of a bombardment ofions.

[0041] In particular, the electric discharge is generated by maintaininga maximum difference in potential between the substrate and acounter-electrode of less than or equal to 1000 V for average powerdensities of between 1 Wcm⁻² and 200 Wcm⁻² per unit surface area of thesubstrate.

[0042] The discharge needed to generate a plasma is obtained bypolarising the substrate negatively with respect to a counter-electrode,either continuously or intermittently. In the former situation, theelectric discharge is continuous and the counter-electrode is an anode.In the second situation, there is an alternating discharge, during whichthe substrate is bombarded with ions only when it is negativelypolarised with respect to the counter-electrode.

[0043] The shape of the voltage signal used for an alternating dischargemay be sinusoidal, simply rectified, square in the case of pulsedcurrents or generally of any other shape. The duration of the positivepart of the voltage signal is not necessarily the same as that of thenegative part over a cycle. The excitation frequency is typicallybetween 1 kHz and 1MHz and is preferably between 10 kHz and 100 kHz. Thesubstrate is advantageously grounded.

[0044] By preference, the time during which the counter-electrode isnegatively polarised with respect to the surface of the substrate isshorter than the time during which it is positively polarised.

[0045] Confining the electrons in the discharge between two or moremagnetic mirrors past which the metal substrate is fed enables thedensity of a plasma generated around the latter to be increased.

[0046] By magnetic mirror in the context of the present invention ismeant a magnetic field of a value that is relatively high compared withan adjacent magnetic field, so that any electrons present in thisadjacent field and moving towards the high-value magnetic field can bereflected into the zone where the lower-value field prevails.

[0047] These magnetic mirrors may be provided in the form of twosolenoids surrounding the metal substrate, for example, or alternativelyas permanent magnets.

[0048] The method proposed by the invention offers a particularlypractical way of treating long or continuous substrates. This being thecase, the substrate is then displaced in its longitudinal directionthrough an enclosure in which the plasma is created.

[0049] In a preferred configuration, the magnetic mirrors are positionedso that they generate an induction field substantially parallel with theaxis of displacement of the substrate, the field becoming stronger inthe direction of each mirror. The magnetic induction field produced atthe centre of a magnetic mirror is preferably at least equal to 5.10⁻³ Tor 50 Gauss.

[0050] As schematically illustrated in FIG. 15, the secondary electronsgenerated as the surface of the substrate 1 is bombarded by thedischarge ions and accelerated in a sheath 16 surrounding the surface ofthe substrate are reflected by the magnetic mirrors until they have lostthe greater part of their kinetic energy in the ionisation process.

[0051] In effect, above a certain critical value of the component of thevelocity vector v_(perp) of an electron perpendicular to the directionof the induction field, which is substantially parallel with thedirection of displacement of the substrate 1, the electron will remainconfined between two adjacent magnetic mirrors.

[0052] By sheath is meant the area of the charging zone, the thicknessof which is generally not more than a few millimetres, which isestablished and naturally separates any surface in contact with aplasma. If this surface is that of an electrode and if this electrode isbrought to a negative potential with respect to that of acounter-electrode or anode, the sheath formed there is known as acathodic sheath. The greater part of the difference in potential appliedbetween this cathode and the counter-electrode or anode will thus bepresent on a level with this cathodic sheath. For this reason, thecathodic sheath is the site from which positive ions are acceleratedtowards the cathode and the site from which any secondary electronsemitted are accelerated towards the plasma following the impact of theions with the cathode.

[0053] If B_(min) represents the minimum magnetic induction fieldbetween two successive mirrors and B_(max) corresponds to the magneticinduction field parallel with the central axis between these twomirrors, generated on a level with each mirror, it can be demonstratedthat for an axial symmetry where B_(min) is parallel with the directionin which the substrate 1 is displaced, an electron will remain confinedbetween these two mirrors if the ratio of the perpendicular componentv_(perp) of the velocity of the electron to its total velocity modulus vis higher than a critical value of sinθ_(cr)=(B_(min)/B_(max))^(1/2)where θ is the angle between the direction of the induction field andthe total velocity v. See for example: [J. R. Roth, Industrial PlasmaEngineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2, pp75-90]. See FIG. 15.

[0054] An electron will escape through the mirrors if the dischargeangle θ is smaller than the critical discharge angle θ_(cr), which ismathematically expressed by the following equation:$\frac{v_{perp}}{v} = {{{\sin \quad \theta} < {\sin \quad \theta_{cr7}}} = \sqrt{\frac{B_{\min}}{B_{\max}}}}$

[0055] Since the electric field E of the sheath 16 is alwaysperpendicular to the magnetic induction field B due to the fact that thelatter is substantially aligned with the axis of displacement of thesubstrate 1 to be treated and the voltage of the sheath 16 is high, theperpendicular component of the velocity vector of the electron v_(perp)is always much higher than its component parallel with the magneticinduction field v_(para) because the latter substantially corresponds tothe mean thermal velocity of the electrons in the plasma, correspondingto a much weaker kinetic energy. The thermal energy of the electron istypically less than 10 eV in this type of plasma. This explains why themagnetic confinement of the secondary electrons remains very effectiveup to the point where, due to inelastic collisions, they lose almost allthe energy gained as they are accelerated in the cathodic sheath 16.These electrons therefore rebound between the magnetic mirrors until theenergy gained in the sheath 16 is exhausted.

[0056] The voltage of the sheath 16 is typically in the order of severalhundred Volt.

[0057] Apart from this longitudinal motion, the electrons also describea motion around the substrate (1), in a direction perpendicular to theinduction field and in a direction perpendicular to the electric field(see FIG. 16). Finally, these electrons therefore describe a spiraltrajectory 17 around the substrate 1 to be cleaned, reboundingcontinuously between the magnetic mirrors until the energy gained due toacceleration in the sheath 16 is exhausted. In effect, a magnetrondischarge is obtained around the substrate 1 to be cleaned or heated,confined between two successive mirrors. Only the electrons whosedischarge angle θ is smaller than the critical angle θ_(cr) can escapefrom the confinement zone, and can do so in a proportion that will againdepend on the ratio of the induction fields B_(min) between the mirrorsB_(max) on a level with the mirrors. In practice, only electrons whichare “thermalised” over the course of numerous collisions will be able toescape through the magnetic mirrors. By “thermalised” electrons is meantelectrons whose kinetic energy is reduced and reaches the mean value ofthe energy of the energy distribution of the electrons in the electricdischarge. Accordingly, the counter-electrodes or the anodes can bepositioned on a level with the magnetic mirrors without any risk ofsecondary electrons being picked up before they are thermalised. It canbe demonstrated that the fraction F_(th) of totally thermalisedelectrons which are trapped between the magnetic mirrors is:

F _(th)=(1−B _(min) /B _(max))^(1/2)

[0058] (J. R. Roth, Industrial Plasma Engineering, Vol. 1, IOPPublishing (1995), ISBN 0 7503 0318 2).

[0059] It is possible to use different specific configurations toconfine the electrons and hence to confine the plasma at the externalsurface of the substrate.

[0060] A first method is based on confining the discharge between twoadjacent mirrors in the direction perpendicular to the axis along whichthe substrate is displaced, due to the presence of an axial magneticfield, as illustrated in FIGS. 1, 5 and 7. The latter enables thecoefficient of ambipolar diffusion D_(perp(a)) of the plasma in thedirection perpendicular to the feed axis to be reduced to the value ofthe coefficient of diffusion D_(perp(b)) of the electrons in this samedirection, this value being much lower than the value of the coefficientof ambipolar diffusion of a non-magnetised discharge D_(a). See [M. A.Lieberman, A. J. Lichtenberg in Principles of Plasma Discharges andMaterial Processing, Wiley Interscience, ISBN 0471005770, NY (1994), pp129-145].

[0061] The presence of the magnetic induction field in the direction ofdisplacement of the substrate in effect limits the diffusion of theplasma in the direction disposed transversely to the direction ofdisplacement of the substrate, perpendicular to the lines of themagnetic induction field, by forcing the electrons to rotate about thesame field line until they collide with another particle, for example anatom or a gas molecule. This configuration also has the advantage ofgenerating a magnetron discharge around the treated substrate, theeffect of which is to further improve confinement of the plasma aroundthe substrate. This configuration can be set up using two magneticmirrors generating a field in the same direction.

[0062] In order to generate such a magnetron discharge, the magneticinduction field parallel with the feed axis of the substrate isadvantageously at least equal to 10⁻³ T (1 T=1 tesla) and is preferablybetween 10⁻³ T and 0.25 T in the treatment zone.

[0063] In a preferred embodiment of the invention, the magneticinduction field is heightened by other means in the direction in whichthe substrate is displaced, for example by incorporating a thirdsolenoid between the two magnetic mirrors or by the presence ofpermanent magnets between the mirrors.

[0064] It is also possible to provide the treatment zone with only onemagnetic induction field parallel with the direction of displacement ofthe substrate, obviating the need to install magnetic mirrors, if thetreatment zone is long enough for plasma losses to be kept low at theinlet and outlet of this zone relative to the plasma generated in thissame zone.

[0065] A second method is to confine the secondary electrons in thespace bounded by two mirrors by using a specific configuration of thelatter, known under the name of a magnetic cusp, as illustrated in FIGS.3 and 9. In this instance, the fields produced by two adjacent mirrorsparallel with the feed axis are inverted at a cusp, the effect of whichis to produce a confinement zone for the electrons both in the directionof displacement of the substrate and in the direction perpendicular tothis direction.

[0066] In practical terms, this configuration has the advantage ofgenerating a magnetic mirror around the substrate in the treatment zonein addition to the magnetic mirrors through which the product passes.The component of the magnetic induction field parallel with the feedaxis disappears at the cusp point. See [J. R. Roth, Industrial PlasmaEngineering, Vol. 1, IOP Publishing (1995), ISBN 0 7503 0318 2, p 83].

[0067] A third method uses a multi-polar magnetic confinement on asurface of revolution surrounding the substrate to be treated, betweenthe two magnetic mirrors through which the latter passes. Thismulti-polar confinement may be produced by a juxtaposition of permanentmagnets, as illustrated in FIG. 11 for example.

[0068] Another known method is to pass currents in reverse directions(FIG. 13) through closed solenoids surrounding the product and mutuallyspaced apart. These solenoids may be reduced to wires surrounding theproduct, through which currents are successively passed in reversedirections. Seer [J. R. Roth, Industrial Plasma Engineering, Vol. 1, IOPPublishing (1995), ISBN 0 7503 0318 2, p. 88-90] et [M. A. Lieberman, A.J. Lichtenberg in Principles of Plasma Discharges and MaterialProcessing, Wiley Interscience, ISBN 0471005770, NY (1994), pp 146-150].

[0069] A significant advantage of the method proposed by the inventionis that, unlike conventional magnetron etching processes, it enables amagnetron discharge to be generated around substrates withcross-sections of different shapes. The method is suitable for cleaningsubstrates in the form of T, I or U-shaped beams, wires or bundles ofwires, strips, sheets, etc., for example. It may also be used to treatthe edges of sheeting. The method proposed by the invention does not infact require the presence of permanent magnets positioned to the rear ofthe substrate, nor does it require any complicated devices to adapt todifferent widths of strips. See for example: EP535568, EP780485,EP879897, D136047, EP878565, EP908535, [S. Schiller, U. Heisig, K.Steinfelder and K. Gehn, Thin Solid Films, 51(1978)191].

[0070] The method may be used to clean the external surface of a productby bombarding it with ions from the plasma. This being the case, toensure a highly kinetic cleaning action, it is preferable to operate ata sufficiently low pressure, preferably at a pressure below 1 Pa, toprevent the ions accelerated in the cathodic sheath from collidingbefore they hit the surface of the substrate.

[0071] In order to clean the substrate, an electric discharge isgenerated in a gas which is advantageously maintained at a pressurebelow 1 Pa and preferably between 1 Pa and 0.01 Pa in the treatmentzone.

[0072] If the process is used as a means of annealing a metal productand it is desirable to prevent the external surface from being erodeddue to ion bombardment, it is preferable to operate at a pressure suchthat the ions collide with the atoms or gas molecules a number of timesas they are accelerated in the cathodic sheath. The surface of thesubstrate is eroded to a significantly lesser degree because the energyis applied to it by a larger number of particles. In practice, theparticles do not have enough kinetic energy to pulverise the superficialatoms when they arrive at the external surface of the metal product. Aheating process is generally operated at a pressure above 1 Pa, andpreferably at a pressure in the order of 10 Pa.

[0073] To heat the substrate, an electric discharge is produced in a gaswhich is advantageously maintained at between 10 Pa and 1000 Pa.

[0074]FIG. 1 illustrates a first embodiment of the device proposed bythe invention, in which a substrate 1 is displaced along an axis ofdisplacement 2 in the direction of arrow 12 through a tank 3,constituting a vacuum chamber, connected to a pumping unit 4. Thepumping unit 4 enables a gas in the tank 3 to be kept at a low pressure.A tube 5 passes through the wall of the tank 3 to enable a gas or agaseous mixture needed for cleaning or heating to be injected into thelatter.

[0075] In this particular case, the device is provided with six coaxialsolenoids 6 (A,B,C, D, E, and F) disposed in the interior of the tank 3,which enable a magnetic induction field B to be created at the centre ofthe solenoids 6, substantially parallel with the displacement axis 2.

[0076] The general pattern of the lines of the magnetic induction fieldcreated by the solenoids 6 is indicated by line 8. The interior of eachof the solenoids 6 is provided with a counter-electrode 9 with arectangular or cylindrical cross-section so that it surrounds the entiresurface of the substrate 1.

[0077] Mounted in the gap between the solenoids 6 and thecounter-electrodes 9 is an electrostatic confinement enclosure 11providing electrical insulation against the walls of the tank 3. Thiselectrostatic confinement enclosure 11 is electrically isolated from thesubstrate 1 and the tank 3 and is therefore kept at a floating potentialwith respect to the potential of the substrate 1 and with respect to thepotential of the tank 3. This avoids creating a discharge on the wallsof the tank 3 and the solenoids 6 are protected from the substanceremoved from the substrate 1 during cleaning. This enclosure 11 alsoenables any metal torn away from the substrate by the bombardment ofions to be recovered. It is preferably tubular in design.

[0078] A discharge 10 is produced between the substrate 1 and thecounter-electrodes 9. The magnetic induction field B generated by thesolenoids 6, through which electric currents are fed in a same directionas indicated by the orientation of arrow 7 perpendicular to the plane ofFIG. 1, is essentially parallel with the axis of displacement 2.Consequently, the orientation of the induction field B is substantiallyconstant. The intensity of the latter varies along the axis ofdisplacement 2. Arrow 7 represents the current vector seen from one ofits ends.

[0079] The induction field is at its maximum and at a value of B_(max)on a level with each solenoid and is at a minimum (B_(min)) between twosuccessive solenoids 6, as clearly illustrated in FIG. 2.

[0080] Consequently, the electrons are generally confined between twosuccessive solenoids as long as the discharge angle θ remains greaterthan the critical discharge angle θ_(cr) The solenoids are close enoughto permit the presence of a minimum induction field B_(min) that is notzero parallel with the displacement axis 2. A magnetron discharge isproduced between two successive solenoids around the product to betreated as a result.

[0081] The embodiment of the invention illustrated in FIG. 3 differsfrom the previous embodiment due to the fact that a discharge isproduced between three solenoids 6 through which a current is passedsuccessively in reverse direction. This being the case, the magneticinduction field 8 remains substantially parallel with the displacementaxis 2 only on a level with the solenoids 6. Between two successivesolenoids, it is disposed transversely to the displacement axis 2. Themagnetic induction field 8 is therefore at its maximum, in terms ofabsolute value, at the centre of the solenoids A, B and C. The fieldparallel with the axis of displacement 2, on the other hand, is rapidlydisappears between two solenoids in the vicinity of the product, asillustrated in FIG. 4. This configuration enables a magnetic mirror tobe formed around the product 1 between two successive solenoids, asclearly illustrated in FIG. 3, due to the concentration of field lines 8around said product between two successive solenoids.

[0082] Consequently, the plasma 10 is also confined in a directionperpendicular to the axis 2 because the intensity of the magneticinduction field in this direction increases due to the field inversionproduced by 2 successive solenoids. The plasma is therefore confined ina zone bounded by two successive solenoids 6, which on the one hand formmagnetic mirrors perpendicular to the displacement axis and on the othera magnetic mirror surrounding the substrate 1, the magnetic mirrorsurrounding the substrate 1 being generally of any section andconforming to the shape of the product, i.e. rectangular in the case ofa strip and cylindrical in the case of a beam or wire, being disposedbetween two successive solenoids.

[0083]FIG. 4 clearly illustrates the presence of a point at which themagnetic induction field B parallel with the displacement axisdisappears and is reversed on the axis 2.

[0084] The embodiment of the invention illustrated in FIG. 5 correspondsto the preferred situation where three successive solenoids A, B and Care fed with current in the same direction and generate a magnetic fieldin the same direction.

[0085] A magnetron discharge around the substrate 1 is produced betweentwo successive solenoids 6. The mirrors A and C and the third solenoid Benable a magnetic induction field to be produced parallel with the axisof displacement 2. This configuration is particularly practical becauseit generally enables a magnetron discharge to be generated around thesubstrate 1 across any length under relatively uniform conditions with ahigh density of plasma. Using solenoids A, B and C with rectangularsections enables metal strips such as wires, beams or tubes to betreated, for example.

[0086] The electrons from the discharge are confined to the interior ofsolenoid B by the magnetic mirrors formed by solenoids A and C as longas the output angle θ is greater than the critical angle θ_(cr) (cfsupra). The intensity of the magnetic induction field parallel with theaxis of displacement 2 is at is maximum at the centre of solenoids A andC and at a minimum in the middle of solenoid B, as illustrated in FIG.6.

[0087] Successive assemblies of this type can be set up one after theother so that each mirror (solenoid A or C) is followed by a solenoid oftype B, thereby permitting a magnetron discharge in a sequence of thetype: ABABABA.

[0088] The configuration of the device proposed by the inventionillustrated in FIG. 7 is a configuration of the magnetic bottle type andis n not made up of solenoids through which an electric current ispassed in the same direction as illustrated in FIG. 1, but of permanentmagnets 13 magnetised in a direction parallel with the direction inwhich the product 2 is displaced.

[0089] Three permanent magnets 13 are mounted in the direction ofdisplacement 2. The north pole of one of the magnets is disposed facingthe south pole of the adjacent magnet. As a result, the magnetic fieldbetween two adjacent magnets is aligned in the same direction. It issubstantially parallel with the axis of displacement 2 and exhibits aminimum value at the centre of the zone between two successive magnets,as illustrated in FIG. 8. The secondary electrons of the discharge 10are reflected by the mirrors constituted by the magnetic poles of themagnets, where the magnetic field is at is maximum and equal to B_(max).The discharge is confined between each pair of successive magnets. Amagnetron discharge is formed around the substrate in each zone,disposed between two successive magnets. Clearly, the return of themagnetic field forces an reversal of the direction of the inductionfield in the free passage between the permanent magnets, enabling theproduct to be passed through. This configuration is particularlypractical as a means of treating metal wires.

[0090] Another embodiment of the device proposed by the invention,illustrated in FIG. 9, differs from that of FIG. 7 due to the fact thatthe three permanent magnets 13 are mounted with the north pole of one ofthe magnets aligned facing the north pole of the adjacent magnet andconversely the south pole facing the south pole of the adjacent magnet.Accordingly, a cusp can be obtained in the component parallel with theinduction field in the feed direction 2, as with the device illustratedin FIG. 3. The presence of a point may be seen, at which the sign of thecomponent parallel with the axis 2 of the induction field is reversedbetween two successive magnets, as illustrated in FIG. 10.

[0091] The plasma 10 is confined along the axis 2 between the twomutually facing magnetic surfaces of identical polarity. The plasma 10is radially confined due to the formation of a magnetic mirror on asurface surrounding the product, closed by the concentration of fieldlines B moving away from the axis 2 between two successive magnets.

[0092] In the embodiment of the device proposed by the inventionillustrated in FIG. 11, the discharge is produced between two magneticmirrors A and C, which enables the electrons to be axially confined. Theplasma is confined around the product by means of a series of permanentmagnets 14 disposed around the electrostatic enclosure 11. These magnetsmagnetically push the electrons and hence the plasma from this enclosure11 back towards the product 1.

[0093] In effect, these permanent magnets 14 constitute a series ofmagnetic mirrors, which force the electrons back in the direction of thesubstrate 1. It goes without saying that the magnets may be disposed indifferent layouts, such as those described in the background literature[J. R. Roth, Industrial Plasma Engineering, Vol. 1, IOP Publishing(1995), ISBN 0 7503 0318 2]. This configuration is particularlypractical if it is desirable to avoid the presence of any magnetic fieldin the confinement zone of the plasma 10. As illustrated in FIG. 12, themagnetic field is negligible on a level with the displacement axis 2between the magnetic mirrors A and C. This configuration also enablesplasmas 10 to be produced in zones of a large volume.

[0094] The configuration of the device illustrated in FIG. 13 isidentical to the one above except for the means used to generate themagnetic field at the surface of the enclosure 11. In this case, thepermanent magnets are in fact replaced by a series of solenoids 15through which currents are successively passed in opposite directions.

[0095] As illustrated in FIG. 14, the variation in the magneticinduction field of the device illustrate in FIG. 13 is the same as thatshown in FIG. 12.

[0096]FIG. 15 illustrates the discharge and the behaviour of theelectrons in the treatment zone disposed between two magnetic mirrors,where the magnetic induction field B is equal to B_(min). The magneticmirrors are disposed in planes perpendicular to the displacement axis 2of the substrate 1. The latter is negatively polarised with respect tothe counter-electrode 9.

[0097] The formation of a cathodic sheath 16 may be seen at the surfaceof the substrate 1. Inside this sheath 16, the electric field E enablesthe ions to be accelerated towards the substrate surface. On impact ofthe latter, the secondary electrons which are emitted are acceleratedtowards the plasma by the same field, as a result of which theperpendicular component v_(perp) of their velocity is on average muchhigher, by reference to the direction of the field B, than the parallelcomponent v_(para) of this same velocity. The angle of confinement θ istherefore greater than the angle of critical confinement θ_(cr) for aslong as a significant part of their energy is not lost in inelasticcollisions. As a result, these electrons are reflected alongtrajectories 17 in the discharge, provided this condition is met. Thecounter-electrode 9 enables electrons which have lost their kineticenergy and which have therefore been thermalised to be returned towardsthe generator, not illustrated in the drawing.

[0098] The counter-electrode may operate as an anode with continuouscurrent or intermittently as a cathode and as an anode in the case ofalternating current. This being the case, several means may be used toprevent the latter from being attacked by ions when negatively polarisedwith respect to the product.

[0099] In a first means, the anodes are positioned only on a level withthe magnetic mirrors, i.e. in a zone where the plasma density isnormally lower than between two mirrors, because it is here that theelectrons are pushed back.

[0100] A second approach is to reduce the time during which thecounter-electrode is negatively polarised compared with the time duringwhich it is positively polarised. In effect, the time during which thesubstrate is negatively polarised may be much longer than the timeduring which it is positively polarised because the electrons aredisplaced more quickly than the ions due to their much lower mass.Consequently, it is possible to use generators with pulsed currents oralternatively generators with currents that are simply rectified (notsmoothed). The advantage of using a discharge with alternating currentis the sharp reduction in the likelihood of “unipolar” arcs appearing onthe treated surface due to electric neutralisation of charges forming atthe surface of dielectric impurities such as grease and oxides stillpresent on the surface of the product to be treated. These charges formnaturally during ion bombardment and are neutralised by the flow ofelectrons when the product is positively polarised with respect to thecounter-electrode.

[0101] Typically, an alternating current is used, at a frequency ofbetween 10 kHz and 100 kHz.

[0102] If using continuous current, the anode may be positionedanywhere, generally speaking. By preference, the substrate is kept atthe ground potential of the installation.

[0103]FIG. 16 illustrates a section in a transverse direction,perpendicular to the direction of displacement 2. The magnetic field Bis therefore perpendicular to the plane of the page. This drawingillustrates how the electrons are displaced around the substrate 1 alonga trajectory 17 under the combined influence of the magnetic field andthe electrical field of the sheath 16. A magnetron discharge istherefore formed around the substrate, as illustrated in FIG. 16. As aresult, the process naturally adapts to any variation in the thickness(th) and width (w) of the substrate 1.

[0104] The electrons rotate about the field lines, whilst the ionsbombard the substrate 1. The trajectories 17 of the electrons are aresult of the fact that the magnetic induction field is disposed in thedirection perpendicular to the electric field of the sheath, which isalways aligned perpendicular to the substrate surface.

[0105] All the magnetic confinement devices may be used either disposedin the interior of (as illustrated in the various drawings) orexternally to the tank 3. In the latter case, the tank 3 must be madefrom a non-ferromagnetic material, such as stainless steel or aluminium,to enable the magnetic induction field to penetrate the interior of thetank 3.

[0106] For the same reason, the confinement enclosure 11 must also bemade from a non-ferromagnetic material.

[0107] Examples of practical applications

[0108] 1. Cleaning Sheets of Hot-Rolled Mild Steel

[0109] A device for cleaning sheets of hot-rolled mild steel was set upusing the configuration illustrated in FIG. 5.

[0110] In this device, the three solenoids A, B and C withrectangular-shaped cross-sections perpendicular to the axis ofdisplacement 2 are of identical dimensions. The length of each solenoidis 400 mm, whilst the mean dimension of the rectangular section is 400mm by 1000 mm. Each solenoid comprises a winding of copper wire.

[0111] The two outer solenoids constituting the magnetic mirrors A and Ceach consume an electric power of 2.240 kW. The central solenoid issupplied with a continuous current of 0.7 A at 800 V, i.e. an electricpower consumption of 560 W. The total electric power consumption neededto generate the induction field B is therefore approximately 5 kW. Thethree solenoids are disposed in sealed carcasses that are permeable tothe magnetic field surrounding the product in the interior of the tank3.

[0112] This configuration enables a maximum field B_(max) of 5.10⁻² T tobe obtained and a minimum induction field B_(min) equal to 2.5 10⁻² T inthe central zone where a magnetron discharge is produced around thesubstrate in the argon, maintained at a pressure of 0.5 Pa (5 mbar).

[0113] Under these conditions, the voltage between anode and cathode isin the order of 500 V for an electric current of 304 A, i.e. an electricpower consumption of 152 kW in the discharge.

[0114] This results in an average power in the order of 19 W/cm²dissipated at the surface of the substrate, constituting the cathode ofthe system, due to a bombardment of argon ions, which corresponds to anelectronic density of 1.14×10¹² cm⁻³ in the plasma. The criticaldischarge angle of the electrons θ_(cr) is 45° under these conditionsbecause sinθ_(cr)=(250/500)^(1/2).

[0115] The secondary electrons accelerated in the sheath are thereforelargely confined by the magnetic mirrors prior to inelastic collisions,the discharge angle θ immediately after acceleration effectively being84°, which is higher than 45°. It may also be demonstrated that, underthese conditions, 71% of the “thermalised” electrons are confinedbetween the two magnetic mirrors A and C.

[0116] The return of current towards the counter-electrodes positionedon a level with the mirrors A and C is therefore due to electrons whosedischarge angle is smaller than 45°, which corresponds to 29% of thepopulation of “thermalised” electrons. The gyration radius of thesecondary electrons accelerated at 500 V is 3 mm and therefore exceedsthe value of the thickness of the cathode sheath, which is in the orderof 0.5 mm at the surface of the substrate.

[0117] Having been accelerated in the sheath, and without undergoing anycollision during this acceleration, the ions bombard the surface of thesubstrate at a virtually normal incidence.

[0118] Four devices of the type described above enabled the sheet to becleaned sufficiently at a speed of 100 m/min to ensure excellentadhesion by a deposit of zinc of a thickness 7 μm by “ion plating” onthe two faces of a sheet one millimetre thick, the temperature of whichwas not more than a hundred degrees centigrade after treatment.

[0119] The sheet was kept grounded whilst the counter-electrodes weresupplied with a current pulsed at 40 kHz. The latter served as theanodes of the system during the greater part of the voltage cycle.During this anode phase of the counter-electrodes, the surface of thesheet was bombarded with argon ions, whereas the short cathode phase ofthe counter-electrodes enabled the positive charges at the surface ofthe sheet to be neutralised, thereby ensuring a discharge withoutelectric arcing.

[0120] The same device enabled sheets of lesser widths and differentthicknesses to be treated without having to make any modifications tothe device other than adapting the electric power consumption. A seriesof wires or parallel strips could then be treated without anymodification to the cleaning process other than adapting the electricalpower to suit the product being treated.

[0121] 2. Heating a Cold-Rolled Steel Sheet in an Annealing ProcessUnder Vacuum by Cold Plasma.

[0122] The same device as that described in point one above, comprisingfour units of three successive solenoids, each fitted with anodes at thecentre of the magnetic mirrors, enabled a sheet 1 m wide and 0.18 mmthick to be heated to a temperature of 600° C. at a rate of 100 m/min,before cooling at low pressure on metal cooling rollers, which were inturn cooled with water. To prevent excessive erosion of the sheet due toion bombardment, the argon pressure was maintained at 10 Pa. Each of thefour heating units consumed a power of 180 kW, which corresponded to atotal power of 720 kW. The density of power dissipated in the treatmentzone was 22.5 W/cm². The four heating units were distributed over 8 mand the average rate of the rise in temperature was estimated to be 125°C./s. Modifying the number of heating units and the power consumed bythe heating units enabled both the temperature to be obtained and therate of temperature rise to be modified. The same device enabled sheetsof smaller widths and different thicknesses to be treated without anymodification to the device other than adapting the electrical powerconsumption.

[0123] 3. Cleaning a Mild Steel Wire Prior to Coating with Zinc

[0124] A mild steel wire was cleaned using a device of the typeillustrated in FIG. 7. The wire passed through eleven treatment units,each bounded by a pair of permanent magnets NeFeB. The effective lengthof each treatment zone was 10 cm, i.e. a 1.1 m effective treatmentlength in total. A steel wire with a diameter of 5 mm fed at a rate of300 m/min was effectively cleaned prior to coating with zinc by “ionplating” at an argon pressure of 0.5 Pa consuming electrical power inthe order of 15 kW. The mean power density in this case was 90 W/cm² andthe rise in temperature of the wire remained below 40 K.

[0125] As explained in the description, the present invention in allcases has the major advantage of enabling a magnetically confineddischarge to be produced around the external surface of a metalsubstrate as it is fed along, thereby enabling its entire externalsurface to be continuously treated, irrespective of the shape of thesubstrate being treated. No modification is necessary other thanadapting the power.

[0126] In one particularly practical configuration of the methodproposed by the invention, apart from confining the discharge around theproduct, the presence of an induction field parallel with the productenables a high-density magnetron discharge to be produced around thelatter. This being the case, it is clear that the use of magneticmirrors is an effective but not indispensable means of limiting plasmalosses at the inlet and outlet of the treatment zone.

[0127] Naturally, the invention is not restricted to the differentembodiments of the method and device described above and illustrated inthe appended drawings and other variants would also be conceivable,particularly as regards the structure and shape of the tank 3, theposition and shape of the solenoids or permanent magnets and the numberof magnetic mirrors, without departing from the scope of this invention.

1. Treatment method, in particular for cleaning and/or heating a metalsubstrate (1) fed in a substantially continuous manner through a vacuumchamber (3), having a treatment zone in which an electric discharge(10), i.e. a plasma, and a magnetic field are generated in a gasmaintained at a pressure below atmospheric pressure, between at leastthe substrate (1), acting as an electrode, and at least onecounter-electrode (9) to enable the substrate (1) to be bombarded by theions produced in the electric discharge (10), characterised in that aconfining magnetic induction field is produced entirely around thesubstrate (1) in the treatment zone so that the electric discharge (10)is also confined to the area all around the substrate (1) in thistreatment zone due to the confinement of electrons released in theelectric discharge (10).
 2. Method as claimed in claim 1, characterisedin that a magnetic induction field is generated in said treatment zonesubstantially parallel with the axis of displacement (2) of thesubstrate (1) through this treatment zone to enable a magnetrondischarge to be formed around the substrate (1) by a circulation ofelectrons from the discharge along trajectories (17) extending aroundthe substrate (1).
 3. Method a claimed in one or the other of claims 1and 2, characterised in that a magnetic induction field is produced, thecomponent of which parallel with the displacement axis (2) is at leastequal to 10⁻³ T (10 Gauss) and preferably between 10⁻³ T (10 Gauss) and0.25 T (2500 Gauss) in said treatment zone.
 4. Method as claimed inclaim 1, characterised in that at least one magnetic mirror is producedentirely around the substrate in the treatment zone.
 5. Method asclaimed in claim 4, characterised in that a magnetic induction field isgenerated essentially transversely to the axis of displacement (2) ofthe substrate (1), the intensity of which increases by at least a factorof two from the substrate (1) to the magnetic mirror.
 6. Method asclaimed in any one of claims 1 to 5, characterised in that at least twomagnetic mirrors through which the substrate (1) is passed are set upand define the inlet and the outlet of the treatment zone, in order toconfine the electric discharge in a direction substantially parallelwith the axis along which the substrate (1) is displaced through thetreatment zone.
 7. Method as claimed in claim 6, characterised in that amagnetic induction field is formed by said magnetic mirrors, which issubstantially parallel with the displacement axis of the substrate (1),the intensity of which decreases at an increasing distance from eachmirror towards the centre of the treatment zone.
 8. Method as claimed inany one of claims 6 and 7, characterised in that a magnetic inductionfield is produced at the centre of the magnetic mirrors that has a valuehigher than any value of the magnetic induction field in a directionsubstantially parallel with the axis of displacement through thetreatment zone and preferably at least equal to twice the value of theminimum magnetic induction field in the vicinity of the substrate (1)inside the treatment zone.
 9. Method as claimed in any one of claims 6to 8, characterised in that the maximum induction field produced by amagnetic mirror is at least equal to
 5. 10⁻³ T (50 Gauss).
 10. Method asclaimed in any one of claims 1 to 9, characterised in that a continuousvoltage is applied between the substrate (1) acting as an electrode andthe counter-electrode (9) so that the substrate (1) is negativelypolarised with respect to this counter-electrode (9).
 11. Method asclaimed in any one of claims 1 to 9, characterised in that analternating voltage is applied between the substrate (1) acting as anelectrode and the counter-electrode (9).
 12. Method as claimed in claim11, characterised in that an alternating voltage is applied between thesubstrate (1) and the counter-electrode (9) at a frequency of between 1kHz and 1 MHz and preferably between 10 kHz and 100 kHz.
 13. Method asclaimed in one or the other of claims 11 and 12, characterised in thatthe counter-electrode (9) is negatively polarised with respect to thesubstrate (1) for a shorter time than the time during which it ispositively polarised.
 14. Method as claimed in any one of claims 1 to13, characterised in that in order to heat the substrate (1), anelectric discharge is produced in a gas maintained at a pressure higherthan or equal to 1 Pa (0.01 mbar) and preferably between 10 Pa (0,1mbar) and 1000 Pa (10 mbar).
 15. Method as claimed in any one of claims1 to 13, characterised in that in order to clean the substrate (1) bybombarding it with positive ions, an electric discharge is generated ina gas maintained at a pressure below 1 Pa (0.01 mbar) and preferablybetween 1 Pa (0.01 mbar) and 0,01 Pa (10⁻⁴ mbar) in said treatment zone.16. Method as claimed in any one of claims 1 to 15, characterised inthat the electric discharge is produced in a noble gas such as argon orhelium and/or a molecular gas such as hydrogen, nitrogen, oxygen and/orhydrocarbon compounds.
 17. Method as claimed in any one of claims 1 to16, characterised in that an electric discharge is produced between thecounter-electrode (9) and the substrate (1) whilst maintaining a maximumdifference in potential between the counter-electrode (9) and thesubstrate (1) of less than or equal to 1000 V for mean power densitiesof between 1 Wcm⁻² et 200 Wcm⁻² per unit of surface area of thesubstrate (1).
 18. Device for treating a metal substrate, in particularfor operating the method as claimed in any one of claims 1 to 17,comprising a vacuum chamber (3) through which a metal substrate (1) isfed in a substantially continuous manner, a treatment zone in which anelectric discharge is generated, magnetic means for confining electronsproduced in the electric discharge and at least one counter-electrode(9) disposed facing the substrate (1) acting as an electrode andenabling this electric discharge to be generated, characterised in thatthe magnetic confinement means are disposed relative to the substrate(1) so that the ions produced in the electric discharge can bemaintained entirely around the substrate (1) and thus bombard thesurface of the latter.
 19. Device as claimed in claim 18, characterisedin that at least one magnetic mirror is disposed around the treatmentzone through which the substrate (1) is fed, so as to reflect and sendback to the substrate (1) the electrons moving away from the latter,thereby confining the electric discharge to the area around thesubstrate (1) inside this zone.
 20. Device as claimed in claim 19,characterised in that said magnetic mirror is set up by a juxtapositionof at least two successive solenoids (6) past which the substrate (1)can be fed and disposed along the axis of displacement of the substrate(1), through which electric currents are successively passed in oppositedirections.
 21. Device as claimed in claim 19, characterised in thatsaid magnetic mirror is set up by means of at least two successivemagnets magnetised in opposite directions along the direction ofdisplacement of the substrate (1) and having an opening through whichthe substrate (1) can be passed.
 22. Device as claimed in claim 19,characterised in that said magnetic mirror is made up of a series ofmagnets disposed around the treatment zone on either side so as to beable to reflect the electrons towards the substrate, and magnetised in adirection disposed transversely to the direction of displacement of thesubstrate (1).
 23. Device as claimed in one or the other of claims 21and 22, characterised in that the magnets are made up of an assembly ofmagnets magnetised in the same direction and disposed in the samedirection.
 24. Device as claimed in any one of claims 18 to 23,characterised in that it has at least one solenoid (6) with an axissubstantially parallel with the axis of displacement of the substrate(1) around the treatment zone.
 25. Device as claimed in any one ofclaims 18 to 24, characterised in that it has at least two magneticmirrors bounding the treatment zone in the direction of displacement ofthe substrate (1).
 26. Device as claimed in claim 25, characterised inthat it has at least two successive magnets magnetised in the samedirection disposed in the direction of displacement of the substrate (1)and having an opening through which this substrate (1) can be passed(1).
 27. Device as claimed in claim 25, characterised in that it has atleast two successive magnets magnetised in opposite directions in thedirection of displacement of the substrate (1) and having an openingthrough which the substrate (1) can be passed.
 28. Device as claimed inclaim 25, characterised in that it has at least two solenoids (6)capable of creating a magnetic induction field in the direction ofdisplacement of the substrate (1) and being disposed about the axis ofdisplacement of this substrate (1) so that the latter can be fedthrough.
 29. Device as claimed in any one of claims 18 to 28,characterised in that said counter-electrode (9) is disposedsubstantially in the vicinity of the magnetic mirrors, past which thesubstrate (1) can be fed (1).
 30. Device as claimed in any one of claims18 to 29, characterised in that the vacuum chamber (3) is a confinementenclosure (11) bounded by walls of a non-ferromagnetic material, whichis electrically insulated from the substrate (1) and thecounter-electrode (9).
 31. Device as claimed in claim 30, characterisedin that the magnetic confinement means are disposed externally to theconfinement enclosure (11) between the external wall of the latter andthe internal wall of the vacuum chamber (3).
 32. Device as claimed inany one of claims 18 to 30, characterised in that the magneticconfinement means are disposed externally to the vacuum chamber, inwhich case the walls thereof are made from non-ferromagnetic materials.